PolynomialRing and from __future__ import unicode_literals

savecancel

There are lots of places in the Sage codebase, including PolynomialRing, where someone does an isinstance(something, str) when they should do isinstance(something, basestring), where basestring is the shared parent of str and unicode.

In this case:

elif isinstance(arg1, str) or (isinstance(arg1, (list,tuple)) and len(arg1) == 1):

Because of how the branching works, it looks like you could also get away with making the names a list:

sage: from __future__ import unicode_literals
sage: R = PolynomialRing(QQ, 'x')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/Applications/sage/devel/sage-main/sage/<ipython console> in <module>()

/Applications/sage/local/lib/python2.7/site-packages/sage/rings/polynomial/polynomial_ring_constructor.pyc in PolynomialRing(base_ring, arg1, arg2, sparse, order, names, name, implementation)
    425     if R is None:
    426         raise TypeError("invalid input (%s, %s, %s) to PolynomialRing function; please see the docstring for that function"%(
--> 427             base_ring, arg1, arg2))
    428 
    429     return R

TypeError: invalid input (Rational Field, x, None) to PolynomialRing function; please see the docstring for that function
sage: R = PolynomialRing(QQ, ['x'])
sage:

but this could lead to similar problems elsewhere.